refactor(spec): one candidate producer over one target-eligibility rule

Both hosts routed verbs themselves and both policies enumerated their own
actions, and all four drifted. Web sent `swipes` to scrollable containers only,
so swipe-to-dismiss on a list row was reachable on native and unreachable on
web; the model policy folded gestures its own way and could not reach what the
seeded picker drew.

A host now reports facts about every element and never decides which verb may
act on it: targets.ts acceptsTarget owns that for both. pick.ts builtinCandidates
is the single enumeration, and the model policy reads it through
__sanderlingEnumerateBuiltin__ instead of reimplementing it in Go.

Gesture verbs change with it: scrolls stay vertical over scrollable containers,
swipes go free-form in all four directions from any element with real bounds.

Claude-Session: https://claude.ai/code/session_01Fj4wJUikdABuMQEETwW55J

pkg/spec/test/defaults.test.ts

@@ -15,10 +15,14 @@ import type {
 function installRuntime(initialState: State): void {
   const state = { current: initialState };
   const runtime: SanderlingRuntime = {
-    extract: <T>(getter: (s: State) => T): Extracted<T> => ({
-      current: getter(state.current),
-      previous: undefined,
-    }),
+    extract: <T>(getter: (s: State) => T): Extracted<T> => {
+      const handle: Extracted<T> = {
+        current: getter(state.current),
+        previous: undefined,
+        named: () => handle,
+      };
+      return handle;
+    },
     always: () => ({ __sanderlingFormula: true } as Formula),
     now: () => ({ __sanderlingFormula: true } as Formula),
     next: () => ({ __sanderlingFormula: true } as Formula),